DIAGNOSIS An obstruction is blocking the door latch mechanism, or the door seal (gasket) is misaligned or folded over.
FIX Inspect the door's inner edge and the corresponding frame on the microwave body. Look for small food debris, a piece of plastic packaging, or a warped door gasket. Clean the latch area and the grooves with a soft brush. Ensure the gasket lays flat and is not pinched.
IF THAT DIDN'T WORK The door hinge may be misaligned. Check if the door is even on both sides when closed. If one side is higher, the hinge may need adjustment or the mounting screws on the hinge side may be loose. Tightening requires caution to avoid stripping.
HEADS UP Never force the door shut. This can break the primary door switch, which is a safety interlock. A broken switch will prevent the microwave from starting at all.